During the reaction of methyl chloride with silicon a complex mixture of chlorosilanes is produced. This complex mixture is normally separated by fractional distillation. Two of the largest volume chlorosilanes, dimethyldichlorosilane and methyltrichlorosilane, have boiling points sufficiently close that distillation columns of 200 or more plates are often required to obtain pure components. Therefore, the separation of dimethyldichlorosilane and methyltrichlorosilane on a commercial scale is expensive in view of both capital and operational costs.
The separation of other close-boiling chlorosilanes also represent a difficult and challenging problem to the silicone industry. For example, tetrachlorosilane and trimethylchlorosilane form an azeotropic mixture which, naturally, cannot be separated by simple fractional distillation. The separation of methylphenyldichlorosilane and phenyltrichlorosilane is complicated by their high boiling points (&gt;200.degree. C.) as well as the closeness of their boiling points.
One object of the present invention is to provide an alternative method for the separation of close-boiling chlorosilanes which avoids problems associated with the conventional separation techniques currently employed by the silicone industry. Another object of the present invention is to provide a method in which close-boiling chlorosilanes can be separated by using the technique of liquid-liquid extraction. Another object is to provide a method by which close-boiling chlorosilanes, which cannot be separated by conventional fractional distillation, can be separated. Other objects of the present invention will be apparent to those skilled in the art upon consideration of the specification.
In addition to the conventional fractional distillation techniques, other methods have been proposed for the separation of close-boiling chlorosilanes. For example, U.S. Pat. No. 3,007,956 teaches that the separation of close-boiling chlorosilanes can be facilitated by the use of certain dinitrile compounds using extraction techniques. The separation of the azeotropic mixture of tetrachlorosilane and trimethylchlorosilane was not reported in U.S. Pat. No. 3,007,956. The technique of U.S. Pat. No. 3,007,956 has not been employed widely, if at all, in the silicone industry perhaps because of the toxic nature of nitrile compounds in general. The separation of the azeotropic mixture of tetrachlorosilane and trimethylchlorosilane has been carried out by a variety of methods including, for example, the addition of acetonitrile or acrylonitrile. According to U.S. Pat. No. 2,388,575, the nitriles form an azeotrope with tetrachlorosilane which can be distilled from the mixture leaving pure trimethylchlorosilane. U.S. Pat. No. 2,388,575 also reports that solvent extraction is not applicable to the separation of tetrachlorosilane and trimethylchlorosilane because of their extreme reactivity and the slight structural and solubility differences between them.